The clearance between a turbine blade and the turbine housing, known as the tip clearance, is critical to the performance of a turbine. Tip clearances that are too large result in leakages that are detrimental to turbine performance. Tip clearances that are too small may result in friction or wear within a turbine and may cause turbine malfunction. Maintenance of tip clearances at a suitable level is important for efficient and robust operation of steam turbines and gas turbines.
An important factor affecting tip clearances is the thermal expansions of the support structures of turbines. The outer shell of a turbine generally is supported by a structure separate from that supporting the turbine rotor. The outer shell of turbines often have extensions known as shell arms, which rest on standards. Similarly, rotors often are supported in standards on journal bearings. As the turbine heats during operation, gases flowing through the outer shell of the turbine heat the shell. The heat is transmitted through the shell to the shell support structure. Rotor support structures also heat during turbine operation but to a lesser degree than shell support structures.
Known materials used in turbine support structures expand when heated. Because of the differing degrees of heating, turbine shell and rotor support structures constructed of known materials may exhibit differing vertical expansions during turbine operation. The differing vertical expansions can affect tip clearances inside the turbine.
There is a desire, therefore, to provide a material for turbine support structures with minimal or controlled thermal expansion. Use of such a material may avoid the tip clearance problems caused by the thermal expansion of turbine support structures constructed of known materials.